Piezoelectric transducers are characterized in that they have the ability to (1) generate a voltage when stress is applied and/or (2) produce a mechanical force when a voltage is applied. These properties are generally referred to as the xe2x80x9cpiezoelectric effectxe2x80x9d and are found only in a limited number of materials. Specifically, when stress is applied to these materials, displacement of the crystalline structure produces a charge (or voltage) proportional to the strain. Conversely, when an electric field (or voltage) is applied to the material, the shape of the crystalline structure changes, thereby changing the dimensions of the material.
The ability of a piezoelectric transducer to generate a voltage when strained enables its use as a sensor to detect displacement by monitoring the voltage that it generates. Conversely, the characteristic expansion or contraction of the piezoelectric transducer when a voltage is applied enables its use as an actuator to produce a controlled force or movement. Both of these effects are useful for a variety of electronic applications.
Tetragonal-phase barium titanate (BaTiO3) was once the material of choice for fabricating piezoelectric transducers. However, tetragonal-phase barium titanate transducers were largely displaced from use when lead-based piezoelectric materials (e.g., lead zirconate titanate, a.k.a., PZT) were discovered due to the superior performance in terms of the much higher strain levels and the higher piezoelectric coefficient of the lead-based materials relative to traditional tetragonal-phase barium titanate. Quartz is also used as a piezoelectric material. However, the magnitude of the piezoelectric effect exhibited by quartz is negligible in comparison to that exhibited by the lead-based materials that are used.
Notwithstanding the advantages of using lead-based materials, such as PZT, as piezoelectric transducers, use of these materials can cause significant problems. Because lead is toxic, its use often presents serious health and environmental hazards. Additionally, lead has a high vapor pressure and is highly reactive. Consequently, processes for fabricating a material with the desired lead-based piezoelectric composition are very unpredictable and unreliable, and the defect rates of the resulting product are very high.
In contrast to known lead-based piezoelectric transducers, a piezoelectric transducer of this invention has a barium-titanate-based composition in a poled, rhombohedral structure. In preferred embodiments, the piezoelectric material is stable at room temperature (and atmospheric pressure). The rhombohedral structure of the barium-titanate-based material can be stabilized at room temperature by doping the barium titanate with an appropriate concentration of KNbO3, BaHfO3, BaSnO3, BaZrO3 or KTaO3.
The transducer can further include electrical contacts configured to allow for transmission of a voltage to or from the barium-titanate-based piezoelectric material. Where the transducer is used as an actuator, a voltage source coupled with the transducer can supply a voltage to the transducer to generate physical displacement, or strain, in the piezoelectric material. Further, a processor programmed to determine the voltage necessary to produce a particular strain in the barium-titanate-based material and further programmed to instruct the voltage source to generate that voltage can be put in communication with the voltage source to perform this function. In preferred embodiments, the barium-titanate-based material exhibits an absolute peak strain of at least about 0.1% when actuated by a voltage source. Where the transducer is used as a sensing device, a sensor can be coupled with the transducer and used to detect the electric field generated by the barium-titanate-based piezoelectric material under strain. Further, a processor programmed to calculate displacement as a function of the generated electric field can be put in communication with the sensor to calculate the displacement of the barium-titanate-based piezoelectric material.
The apparatus and methods of this invention offer a number of advantages over the prior art. A piezoelectric transducer of this invention offers relatively-high strain levels and piezoelectric constants far higher than those of tetragonal-phase barium titanate and comparable to those of PZT and other lead-based piezoelectrics without the toxicity and unpredictability that plague the use of lead-based materials. Moreover, barium titanate is inexpensive and the know-how required to process it for piezoelectric applications is already in place due to the known use of conventional, tetragonal-phase barium titanate. Further, the addition of a respective percentage of a dopant, such as BaZrO3, BaHfO3, KNbO3 BaSnO3 or KTaO3 within a particular concentration range in accordance with aspects of this invention results in a barium-titanate-based material that can exist in a rhombohedral crystalline structure at or above room temperature and ambient atmospheric pressure, thereby facilitating its use for room-temperature transducer applications.